Emily Watson: 'Sexuality is a big part of who I am'

And then sometimes they're not. The latest three-parter from the BBC, The Politician's Husband, stars David Tennant and Emily Watson as two senior politicians who are married to one another and whose domestic life starts to unravel when the husband's career begins to be eclipsed by that of his more talented wife. If this is a fantasy (and none too flattering) version of Ed Balls and Yvette Cooper's lives, then at least they can be happy with the cast.

It's a mark of how much the star of TV drama has risen that an actor of Emily Watson's stature is involved. Ever since she was nominated for an Oscar for her debut role in Lars von Trier's mesmerising (and disturbing) Breaking the Waves, she's worked with some of Hollywood's most interesting names.

Paul Thomas Anderson (who went on to direct the Oscar-studded There Will Be Blood) wrote Punch-Drunk Love for her; Steven Spielberg cast her in War Horse; Robert Altman picked her to be his world-weary housemaid, Elsie, in Gosforth Park. She was nominated for another Oscar for her role in Hilary and Jackie, the story of Jacqueline du Pré, so to have her popping up in a BBC drama is something of a surprise and a delight.

But then it's as dark and vengeful as a Jacobean revenge tragedy at times, a gripping political psychodrama, and while Ed Balls can dream (and only dream) of having the sex appeal of David Tennant – even aged-up with ridiculous TV character hair (it really ought to have had some lines of its own, it's that arresting) – there is a certain quietly competent Yvette Cooper air that hovers around Emily Watson's performance.

Did Watson study her at all? "No. Not specifically. I mean, I think Paula Milne [the writer] had that couple in mind. But it's not supposed to be them, or anybody specifically. There are quite a lot of frontrunners at the moment who are women, on both sides. So I think that's quite interesting – and what it does to a relationship when both partners want the top job."

She's right. It is quite interesting. And the marriage of the Balls-Coopers is an inherently dramatic starting point in a Borgen-meets-The-Borgias sort of a way. What's more, it's a drama with a backstory: back in 1995, at the height of John Major's government, when Back to Basics was the policy of the day and barely a week went by without some minister being caught with his mistress, Milne wrote The Politician's Wife. It was a huge hit for Channel 4, winning Baftas and Emmys and one of the factors, no doubt, that influenced Watson to take the role, though she says she just read the script. "And I thought: 'Ooh, that's quite tasty.'"

"Tasty" in that Yvette and Ed – or Aiden and Freya, as they're called in this scenario – are ambitious enough and ruthless enough to try and stitch each other up. Despite having two children and a home together, Aiden becoming unhinged when his wife replaces him in the cabinet.

"She's quite an unusual female character," says Watson, "in that she's very sexual, but a faithful wife and mother. And she's very ambitious and political but has the emotional intelligence to play a very subtle hand. It's quite complex, and I just enjoyed playing her. And you know, she had brushed hair and suits and make-up and all that."

Really, I say? You liked the lady-politician viscose blouses?

"But that is true to life. If you look at women in politics, they all dress like men. The currency of your sexuality is not… you can't take it to work. Whereas obviously in my job, it's a big part of what I do. It's a big part of who I am, and there's no denying it. Whereas I think in the political world you have to very strictly leave that at home. And I think any politician who tries to be sexual or feminine is diminished."

It's an interesting observation, particularly given Watson's career. At 46, she says she "plays a lot of mums these days. Which is fine. I am a mum, after all." She has two children, Juliet, seven, and Dylan, four, in real life, and on screen she was, recently, the mother in War Horse. And she's currently commuting back and forth to Berlin, where she's playing another maternal role in The Book Thief. But in many ways her defining role, and the one that first brought her to public attention, was highly sexualised: that of Bess in Breaking the Waves. It was an extraordinary film and Watson was extraordinary in it.

"It's funny," she says, "a few days ago the Book Thief's director, Brian Percival, said to me: 'Oh I watched Breaking the Waves again over the weekend.' And I was like: 'Really? I mean, really?'"

Why?

"It's just so extreme and so powerful and… raw."

It was raw. Watson played the role as if she'd had her skin sheared off, and maybe she had, in a way: it was her first film – "I had nothing to compare it to," she says – and when von Trier told her to open herself up to the role – playing a naive God-fearing Scottish girl whose husband suffers a grisly industrial accident and insists she goes out and sleep with other men – it's exactly what she did.

How does it feel watching it now? Do you still feel like that young, wide-eyed girl?

"Well, I wasn't that wide-eyed, really. I was 28 and I'd been to university, I'd been in the theatre. I just managed to pull it off and make it look like I was. But yes, I look back on that and it was absolutely a shatteringly life-changing experience. It was huge. It was a monumental experience, not just in the making of the film but in the effect that it then had on my life."

It propelled her into the Hollywood elite. And yet, curiously not, in a way. I come across an old cutting that compared her with Kate Winslet – "The only two young British actresses who have genuine heavy- weight status in America," it claimed.

It seems unlikely that you'd bracket the two of them together these days. Watson has made her life in southeast London, not Hollywood, and has been married for nearly two decades. Whereas Winslet has made, let's say, other choices. "There was a point where I was thinking about moving to LA," says Watson. "I physically enjoy the place. But it's the mental life you have to negotiate living there. I just don't think I have the equipment to deal with it."

Her decisions have had their own stresses, though. As a jobbing actor before her big break, she met and married fellow actor Jack Waters, and while her career achieved super-stratospheric lift-off, her husband just never quite got the break. In The Politician's Husband, Watson's character's growing ambition and career focus, coupled with her husband's lack of success, leads to a troubled female guilt/male emasculation dynamic. It's a tricky question to ask, but have there been any parallels?

"It's a very different scenario, because being an artist you're not… It doesn't have that same climbing-a-structure thing; it's much more fluid. And it's very much a part-time job, so when I'm home I'm very content to be the cooker and the washer. Obviously I've had a career that he hasn't had, so that is an imbalance, but as you get older… I just feel that you don't have to judge your life by other people's standards. It's what you make of it. You just have to love the life you're in, really."

These days, she says, the roles she accepts are entirely influenced by the impact they have on her family. "My career is totally controlled by that," she says. "It's a huge part of my decision-making process. Where is it? How long? And how much? Because if it's independent, low budget, no money, eight weeks, fantastic script – sorry. It has to be worth my while leaving for that long. To keep everybody in nannies and school fees."

The geography issue is presumably why she has started to turn up in certain TV dramas. She co-starred with Dominic West last year in the Fred West drama Appropriate Adult, for which she won a Bafta. In the YouTube clip of it she looked genuinely stunned, as opposed to faux oh-my-God! actorly stunned, and admits that she found it incredibly gratifying. "I've just been to so many of those things and never won. It was a bit of a monkey off my back."

But the role she's playing now, she says, "of the multitasking mum", is one she's enjoying. "Though it can be a bit surreal." A day or two ago she'd played a woman waving her husband off to the front in Nazi Germany. "And then I'm driving back from the Polish border to get the late flight from Berlin and then going straight to Juliet's school play in the morning. It can be a bit odd."

Somebody once told her that actors' bodies can't distinguish between fiction and reality, between invented emotion and real. "And I'm sitting there, and I have to remember that there's nothing wrong. It's all good. The Nazis aren't coming. It's just 'London's Burning' on the recorder."